treewide: add license header and enable haslicense linter

Change-Id: I873a8d4082d75e8f813d8a726a41187eea7a065e
Reviewed-on: https://review.monogon.dev/c/monogon/+/3825
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/metropolis/node/core/clusternet/clusternet.go b/metropolis/node/core/clusternet/clusternet.go
index 96a500d..89c5c10 100644
--- a/metropolis/node/core/clusternet/clusternet.go
+++ b/metropolis/node/core/clusternet/clusternet.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 // Package clusternet implements a Cluster Networking mesh service running on all
 // Metropolis nodes.
 //